The mother of a victim of the 2005 London terrorist attacks is suing Rupert Murdoch's media empire after she was told by police that her son's mobile phone is likely to have been targeted by a private investigator working for the News of the World.

Sheila Henry filed a high court writ this week against the paper's owner, News Group Newspapers, alleging that journalists at the tabloid, which closed in July, hacked into a mobile belonging to Christian Small, 28, on the day he was killed by a bomb blast on the London Underground.

Henry left messages on her son's phone on the day of the attacks, in which 52 people died. In common with many of the victims, Small was missing for some time after the initial bomb blasts, and his family were trying to discover where he was.

The news emerged on the same day as fresh revelations in the phone-hacking affair that once again put the conduct of Murdoch's global media group News Corp under intense scrutiny.

The company's UK subsidiary told the high court on Tuesday it had found "tens of thousands" of extra emails that could potentially shed light on the extent of phone hacking at the paper "which the current management were unaware of". They are understood to include correspondence between reporters and senior managers at the News of the World and the Sun.

Mr Justice Vos, the judge overseeing the phone hacking cases, said : "There's some important material in what has already been disclosed. I took the step of looking at some of the material. There's some significant material. I'm sure there's lots more to come."

The high court was also told Scotland Yard has handed a 68-page document to phone-hacking litigants who are pursuing civil cases. It lists the names of News of the World journalists who commissioned Glenn Mulcaire, the private investigator the paper employed, to hack into mobile phones. The fact that the document runs to so many pages suggests Mulcaire acted on the orders of a number of News of the World reporters.

Separately, James Murdoch, 38, son of Rupert and third most senior News Corp executive, was recalled to parliament for a second grilling by MPs over whether he was told three years ago that hacking extended beyond a single "rogue reporter" at the paper. Murdoch's denial was contradicted by the News of the World's last editor, Colin Myler, and former legal head Tom Crone, at the Commons culture, media and sport committee last week.

Meanwhile, a group of News Corp shareholders in America who are suing the firm for corporate negligence widened their action against the company. The investors now allege that "illicit phone hacking and subsequent cover-ups at News of the World were part of a much broader, historic pattern of corruption". The action targets Rupert Murdoch, chief operating officer Chase Carey, and Carey's deputy, James Murdoch.

The case brought by Sheila Henry is the first to be launched by a 7/7 victim or a family member of a victim. The Metropolitan police have warned relatives of a handful of those killed that day that mobile numbers belonging to their deceased relatives were found in Mulcaire's notebooks. It is understood that Mulcaire made a note of Henry's own mobile as well as her son's. The apparent confirmation of the News of the World's willingness to target victims of a terrorist attack brought immediate condemnation.

Labour MP Tom Watson, who has vigorously pursued the hacking allegations, said: "If this is accurate it shows that in the week we commemorated the victims of 9/11, the victims of our own terrorist attack have had their memories insulted in a callous and inhuman way."

A spokeswoman for News International, News Group's parent company, said: "We take very seriously the matters raised in court this morning and we are committed to working with civil claimants to resolve their cases." Henry's claim will be one of half a dozen lead cases heard at trial early next year. If successful it will set a benchmark for the amount of compensation awarded to victims of hacking.

They could include the parents of Milly Dowler, the schoolgirl who was murdered in 2002. The revelation in July that their daughter's phone had been targeted by Mulcaire led to the closure of the News of the World and the resignation of former NI chief executive Rebekah Brooks.

The news comes as the lead investigator in Operation Motorman, a 2006 inquiry by the Information Commissioner's Office into the use of private investigators by newspapers, said that his team were told not to interview journalists involved.

The investigator, a retired police inspector with 30 years experience, accused authorities of being too "frightened" to tackle journalists. "I feel the investigation should have been conducted a lot more vigorously, a lot more thoroughly and it may have revealed a lot more information," he said. "I was disappointed and somewhat disillusioned with the senior management because I felt as though they were burying their heads in the sand. It was like being on an ostrich farm," he told the Independent.